1. Field of the Invention
The present invention relates to a process for producing a compound for rare earth metal resin-bonded magnet in powdery granular form for use as a resin-bonded magnet molded into a predetermined shape by mixing a ferromagnetic alloy powder containing a rare earth element such as R--Fe--B alloy, R--Co alloy, R--Fe--N alloy (where R represents a rare earth element such as Sm or Nd), together with a resin component as a binder.
2. Description of the Related Art
A permanent magnet molded into a predetermined shape is essential as a component of a motor or an actuator to be used in household electric appliances, automobiles, computers, and their peripheries. The scale reduction, energy saving, and weight reduction of these final products are always required, and also motors and actuators are required to have less dimension, less weight, and higher efficiency. In accordance with these needs, a smaller and stronger molded product of a permanent magnetic material is demanded as a component of a motor or an actuator.
Generally, such molded product of a permanent magnetic material can be classified into two kinds: a sintered magnet and a resin-bonded magnet. A sintered magnet is produced by molding a fine powder of a magnetic alloy into a predetermined shape with a high-molecule resin used as a binder, and heating the molded product to remove or burn the binder, followed by sintering. Therefore, the obtained magnet almost consists of a permanent magnetic alloy component alone, and is excellent in magnetic properties, so that it is preferable. However, this requires both the step of molding the magnetic powder and the sintering step. Further, the sintering step requires a comparatively long time, and moreover the production yield is low due to great shrinkage in the sintering step.
On the other hand, a resin-bonded magnet is obtained by using a resin, preferably a reaction-curing resin, as a binder and molding a ferromagnetic alloy powder and the resin into a predetermined shape. In the resin-bonded magnet, the relative ratio of the ferromagnetic alloy decreases due to the presence of the binder. As a result, the resin-bonded magnet is inferior to the sintered magnet in magnetic intensity. However, the resin-bonded magnet has characteristics such that the dimension change is small in the steps after the molding, resulting in a high yield, and it can conform to a variety of product shapes simply by selecting a suitable shape of the mold, thereby achieving an excellent freedom in the shape of the product to be obtained.
A resin-bonded magnet is completed by mixing and covering a ferromagnetic alloy powder with a resin, preferably a reactive resin material into a powdery granular compound for molding, compression-molding the compound in a mold having a predetermined shape, and preferably heating the molded product to reaction-cure the reaction-curing resin material. Conventionally, such a compound has been produced by the following steps.
(1) A ferromagnetic alloy powder of a rare earth metal alloy is ground to have a particle size of less than 250 .mu.m, typically 30 to 250 .mu.m. Then, a reaction-curing resin material and a solvent are added and the mixture is kneaded by means of a kneading apparatus such as a kneader.
(2) The obtained kneaded product composed of the magnetic powder and the reaction-curing resin sometimes contains a large agglomerate and is not suitable for molding as it is. Therefore, the kneaded product is ground and the particles are regulated.
(3) The ground kneaded product is then dried to remove the solvent.
(4) The kneaded product after drying, i.e. a compound, is sieved for classification to collect a compound of less than 250 .mu.m composed of the magnetic powder and the reaction-curing resin.
(5) A lubricant is added and mixed with the compound to complete compound granules for compression molding.
However, the above conventional process involves the following problems.
a) A kneading apparatus such as a kneader having a large shear force is used in preparing the compound. Therefore, the powdery particles of the material alloy is further ground to generate a fine powder, causing change in the particle size distribution of the ferromagnetic alloy powder. As a result, the particle size distribution of the compound granules also changes, leading to decrease in the compression molding property.
b) A rare earth metal alloy, particularly an Nd alloy, especially the fine powder of the above exemplary magnetic alloy, is extremely liable to be oxidized, and the generated oxide decreases the magnetic properties of the resin-bonded magnet after molding.
c) The conventional process requires a long time of 20 to 25 hours for a step of preparing the compound, and moreover it involves a batch process, so that the production efficiency is poor and the cost is high.
The purpose of the present invention is to provide a process for producing a compound that solves the above-mentioned problem of the prior art. Namely, the present invention aims at continuously producing compound granules for a rare earth metal resin-bonded magnet in a short time in which little amount of fine powder is generated due to grinding of the material alloy in preparing the compound, resulting in excellent magnetic properties of the resin-bonded magnet.